Add bp2build support for cpp_std.

This converts cpp_std and gnu_extensions into a -std copt, if cpp_std is
specified or gnu_extensions is false if cpp_std is not specified.

I chose to go with this copts approach because the tradeoff is a much
simpler setting than adding a new attr(s) everywhere that uses features
to set the flag.

This approach limits the number of user-configurable knobs (since users
would then be able to set std in _both_ copts and the new attr). But it
does rely on the user copt overriding the toolchain's default gnu++17
version, which can mean a `-std` flag showing up twice in the action.

Fixes: b/202462232
Test: b build //system/libziparchive:libziparchive
Change-Id: I81dad029059461739b91f318d662e089edb46b84

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